1. Field of the Invention
The present invention relates to a power amplifying module.
2. Description of the Related Art
A mobile communication device such as a mobile phone uses a power amplifying module to amplify the power of a radio frequency (RF) signal to be transmitted to a base station. Such a power amplifying module includes, in addition to an amplifying circuit that amplifies an RF signal, a bias circuit for biasing a transistor forming the amplifying circuit to an operating point. See, for example, Patent Publication JP-A-11-330866.
FIG. 25 is a diagram showing a general configuration of an amplification circuit and a bias circuit. An amplification circuit 2501 amplifies an RF signal (RFIN) input to the base thereof and outputs the amplified RF signal (RFOUT). A bias circuit 2502 is adapted to bias a transistor 2503 forming the amplification circuit 2501 to an operating point and has an emitter-follower configuration. As shown in FIG. 25, in general, a constant voltage VREF controlled to a predetermined level is often applied to a base side of a transistor 2504 of the bias circuit 2502. On the other hand, a power supply voltage such as a battery voltage VBAT is often applied directly to a collector side of the transistor 2504.
In the mobile communication device such as a mobile phone, high linearity is needed for the power amplifying module in order to increase communication speed. However, the battery voltage VBAT may vary significantly within the range of, for example, a little under 3 V to about 5 V. When such a battery voltage VBAT is applied to the collector side of the transistor 2504, a variation in battery voltage VBAT varies a bias output from the bias circuit 2502. Then, the variation in bias output varies the gain of the amplification circuit 2501, reducing the linearity.